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Abstract:

An imaging device determines an in-focus position by contrast autofocus.
The device includes a first optical system including a first lens and a
first image sensor, a second optical system including a second lens and a
second image sensor, a signal processing unit to read an image signal
from at least one of the first image sensor and the second image sensor
and generate an image for display, and a display unit to display the
generated image. In the contrast autofocus, the signal processing unit
reads a part of the image signal from the first image sensor and
calculates a contrast value based on the read part of the image signal,
and generates the image for display from the image signal read from the
second image sensor.

Claims:

1. An imaging device which determines an in-focus position by contrast
autofocus, comprising: a first optical system including a first lens and
a first image sensor; a second optical system including a second lens and
a second image sensor; a signal processing unit configured to read an
image signal from at least one of the first image sensor and the second
image sensor, and generate an image for display; and a display unit to
display the generated image, wherein in the contrast autofocus, the
signal processing unit is configured to read a part of the image signal
from the first image sensor and calculate a contrast value based on the
read part of the image signal, and generate the image for display from
the image signal read from the second image sensor.

2. An imaging device according to claim 1, wherein the signal processing
unit is configured to stop generating the image from the image signal
read from the first image sensor in the contrast autofocus.

3. An imaging device according to claim 1, wherein the signal processing
unit is configured to generate the image in the contrast autofocus by
synthesizing the image signal read from the first image sensor with the
image signal read from the second image sensor.

4. An imaging device according to claim 1, wherein in the contrast
autofocus, the signal processing unit is configured to determine whether
or not a focal length of the first optical system is longer than a
predetermined value, and generate the image from the image signal read
from the second image sensor when the focal length of the first optical
system is longer than the predetermined value.

5. An imaging device according to claim 1, wherein: the first optical
system includes a focus lens; and in the contrast autofocus, the signal
processing unit is configured to determine whether or not a number of
scan steps of the focus lens is larger than a predetermined value, and
generate the image from the image signal read from the second image
sensor when the number of scan steps is larger than the predetermined
value.

6. An imaging device according to claim 1, wherein in the contrast
autofocus, the signal processing unit is configured to determine whether
or not a process time for the contrast autofocus is longer than a
predetermined time, and generate the image from the image signal read
from the second image sensor when the process time is longer than the
predetermined time.

7. An imaging method for determining an in-focus position by contrast
autofocus, comprising the steps of: reading an image signal from at least
one of a first image sensor and a second image sensor, and generating an
image for display; displaying the generated image; and in the contrast
autofocus, reading a part of the image signal from the first image sensor
and calculating a contrast value based on the read part of the image
signal, and generating the image for display from the image signal read
from the second image sensor.

Description:

CROSS REFERENCE TO RELATED APPLICATION

[0001] The present application is based on and claims priority from
Japanese Patent Application No. 2011-117370, filed on May 25, 2011 and
No. 2012-066553, filed on Mar. 23, 2012, the disclosure of which is
hereby incorporated by reference in its entirety.

TECHNICAL FIELD

[0002] The present invention relates to an imaging device having an
autofocus function (hereinafter, AF function).

BACKGROUND ART

[0003] Several types of AF function of an imaging device are known. One of
them is contrast autofocus. The contrast autofocus is to move a focus
lens of a main optical system having an image sensor, calculate a
contrast value of a subject image from the image sensor in each lens
moving step, and determine the position of the focus lens with the
maximal contrast value as an in-focus position.

[0004] There has been a demand for increasing an autofocus speed. A known
technique is to heighten the speed at which a subject image is read for
calculation of a contrast value and thereby increase contrast AF speed.

[0005] To heighten the image read speed, for example, not image data of
the entire image area but that of a part of the image area needed for a
contrast value calculation is read from the image sensor (hereinafter,
partial read process). Assumed that the read speed for the entire image
area is 30 fps (30 images per second), the read speed for the partial
image area can be 120 fps, for example.

[0006] The imaging device includes a main display unit on the back of a
body to display a subject image acquired via an optical system. Users can
take photos while viewing subject images on the display unit. Further,
users can know the optimal shutter timing by visually checking an
indication for the completion of AF process superimposed on the subject
image. Thus, it is preferable to constantly display a subject image on
the display unit during shooting operation.

[0007] However, there is a problem in the partial read process that a
partially read image is not suitable for the user's visual check and
image display may be disordered. In view of this, Japanese Patent No.
4050385 discloses an imaging device to temporarily stop the image display
on the display unit during the contrast AF process and resume it after
the completion of the process, for example.

[0008] Such an imaging device, however, still faces a problem that a
subject image may move to outside the display area when the image display
is resumed after completion of the AF process since the image display is
stopped during the contrast autofocus process in which no image data is
read from the entire image area.

SUMMARY OF THE INVENTION

[0009] The present invention aims to provide an imaging device which can
heighten the contrast AF speed and at the same time continuously display
preview images on the display unit while tracking the motion of a
subject.

[0010] According to one aspect of the present invention, an imaging device
which determines an in-focus position by contrast autofocus, includes a
first optical system including a first lens and a first image sensor, a
second optical system including a second lens and a second image sensor,
a signal processing unit configured to read an image signal from at least
one of the first image sensor and the second image sensor, and generate
an image for display, and a display unit to display the generated image,
wherein in the contrast autofocus, the signal processing unit is
configured to read a part of the image signal from the first image sensor
and calculate a contrast value based on the read part of the image
signal, and generate the image for display from the image signal read
from the second image sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Features, embodiments, and advantages of the present invention will
become apparent from the following detailed description with reference to
the accompanying drawings:

[0012] FIGS. 1A to 1C are a front view, a top view, and a back view of an
imaging device according to one embodiment of the present invention;

[0013]FIG. 2 is a function block diagram of the imaging device in FIG. 1A
to 1C;

[0014]FIG. 3 is a flowchart for the imaging operation executed in the
imaging device by way of example;

[0015]FIG. 4 is a flowchart for the AF process of the imaging operation
by way of example;

[0016]FIG. 5 is a flowchart for the contrast autofocus of the AF process
by way of example;

[0017]FIG. 6 is a flowchart for the sub frame display process of the
contrast autofocus by way of example;

[0018]FIG. 7 shows an example of an image on a display unit in the sub
frame display process; and

[0019] FIG. 8 shows another example of an image on a display unit in the
sub frame display process.

DESCRIPTION OF EMBODIMENTS

[0020] Hereinafter, an embodiment of the present invention will be
described in detail with reference to the accompanying drawings. Wherever
possible, the same reference numbers will be used throughout the drawings
to refer to the same or like parts.

First Embodiment

[0021] A digital camera as an example of the imaging device according to
the present invention is described, referring to FIGS. 1A to 1C. FIGS. 1A
to 1C are a front view, a top view, and a back view of a digital camera
1, respectively.

[0022] The digital camera 1 in FIG. 1A includes, on the front face, a lens
barrel unit 6 including a main optical system having an imaging lens 5, a
stroboscopic unit 7, an optical viewfinder 8 to visually check a subject,
and a sub optical system 16 for external AF process to measure the
distance to a subject.

[0023] In FIG. 1B the digital camera 1 includes, on the top face, a
shutter button 2, a power button 3 and a switch dial 4 for switching
operation modes as shooting mode, reproduction mode.

[0024] In FIG. 1C the digital camera 1 includes, on the back face, an LCD
9 as display unit to display a subject image during shooting operation
and stored image data in reproduction operation, an eyepiece lens 8a of
the optical viewfinder 8, a wide angle (W) zoom switch 10, a telephoto
(T) zoom switch 11, a menu button 12 (MENU) to display menus for setting
operation parameters of the digital camera 1, and an OK button 13. Also,
it includes, on a side, a memory card holder 15 to contain a memory card
14 in which captured images are stored.

[0025] Next, the inner structure of the digital camera 1 according to the
present embodiment is described with reference to FIG. 2. The digital
camera 1 includes a CMOS 104 with a light receiving face to form a
subject image via the imaging lens 5 and output an electric signal or a
digital RGB image signal, a signal processing unit 101 to conduct
predetermined signal processing to the electric signal from the CMOS 104,
and a diaphragm and mechanical shutter unit 18, a motor driver 103 for
the unit 18, and a CPU 102 to control the entire operation of the digital
camera 1.

[0026] The main optical system 17 as a first optical system includes the
imaging lens 5 as a first lens and the CMOS 104 as a first image sensor.

[0027] The digital camera 1 further includes a memory 108 in which
captured images are stored temporarily, a communication driver 107 to
connect the digital camera 1 with an external device via a not-shown
communication interface, a detachable memory card 14 in which captured
image data are stored, a display unit 109 including the LCD 9 and a
display controller to convert image signals from the signal processing
unit 101 into signals displayable on the LCD 9, an operation unit 106
including various buttons as the shutter button 2, switch dial 4, menu
button 2, OK button 13 for users' manipulation.

[0028] Further, the sub optical system 16 as a second optical system
includes a sub lens as second lens and a sub image sensor as a second
image sensor. The digital camera 1 includes the stroboscopic unit 7 to
start and stop light emission under the control of the CPU 102, and a
main condenser 105 for strobe light emission.

[0029] The sub optical system 16 is mainly used for the external AF
process to measure the distance to a subject before the contrast
autofocus in the imaging process.

[0030] The motor driver 103 for the imaging lens 5 and the unit 18 is
controlled by a drive signal from the CPU 102.

[0031] The CMOS 104 includes a light receiving face with two-dimensionally
arranged pixels to convert an optical image into an electric charge and
output it as an electric signal in accordance with an output signal from
the driver 113. RGB color filters are arranged over the pixels to output
digital RGB image signals or RAW-RGB data in association with the three
RGB colors.

[0032] Although not shown, the signal processing unit 101 includes a CMOS
interface (CMOS I/F) to read the RAW-RGB data from a predetermined image
area of the CMOS 104 at a predetermined speed, a memory controller to
control data write and read to/from the memory 108, a YUV converter to
convert the read RAW-RGB data into YUV data, a resize unit to change the
size of image data to one suitable for the size of the display unit or
memory format, a display output controller to control the display output
of image data, a data compressor to convert image data into one in JPEG
form, a medium interface (I/F) for use in image data write/read to/from
the memory card, and a controller for the entire system of the digital
camera 1 according to a control program stored in a not-shown ROM.

[0033] The RAW-RGB data captured by the CMOS I/F, the YUV data converted
by the YUV converter and the compressed image data in JPEG form are
stored in the memory 108. The YUV data is represented by brightness data
(Y), a color difference (U) between the brightness data and blue color
data (B) and a color difference (V) between the brightness data and red
color data (R).

[0034] The sub optical system 16 includes two or more sub image sensors
with two-dimensionally arranged pixels to obtain image data of a subject
from the sub image sensors, calculate parallax in the image data and
calculate the distance to a subject by triangulation.

[0035] Next, an example of a preview operation and a still image shooting
of the digital camera 1 are described. In the still image shooting mode
the digital camera 1 performs the following preview operation.

[0036] Upon a user's power-on with the power button 3 and selection of the
still image shooting mode with the switch dial 4, the digital camera 1 is
operated in the still image shooting mode.

[0037] In the digital camera 1 the CPU 102 outputs a control signal to the
motor driver 103 to move the imaging lens 5 including a focus lens to a
shooting position. The CPU 102 also activates the CMOS 104, signal
processing unit 101, and display unit 109.

[0038] Then, an optical image is formed on the light receiving face of the
CMOS 104 via the imaging lens 5, and an electric signal is output from
the CMOS 104 to an A/D converter 112 via a CDA/PGA 111 in accordance with
the optical image. The A/D converter 112 converts the electric signal to
12-bit RAW-RGB data. The signal processing unit 101 outputs the converted
RAW-RGB data at a predetermined speed via the CMOS I/F and stores it in
the memory 108 as SDRAM.

[0039] The RAW-RGB is read from the memory 108 at certain timing and
converted into YUV data (YUV signal) by the YUV converter to display on
the LCD 9 of the display unit 109. The converted YUV data is stored in
the memory 108 via the memory controller.

[0040] The YUV data is read from the memory 108 to the display unit 109 at
certain timing via the memory controller. Thus, a subject image is
displayed on the LCD 9 in the preview operation.

[0041] During the preview operation, RAW-RGB data corresponding to the
entire image area of the CMOS 104 is output as a single frame of YUV data
with 1/30 second interval and displayed on the LCD 9. The preview
operation continues to display a subject image on the LCD 9 as an
electric viewfinder until a full-press or half-press to the shutter
button 2 is detected.

[0042] In the preview operation a user can visually check a subject image
on the LCD 9. The display unit 109 can output image data as a TV video
signal to display video on an external TV via a video cable.

[0044] The AF evaluation value is an integrated value of the output of a
high frequency component filter or an integrated value of brightness
difference in neighboring pixels, for example. When the imaging lens 5
captures a subject in focus, the edge portion of the subject exerts
highest frequency components. Using this, in autofocus operation the
focus lens of the imaging lens 5 is driven to calculate the AF evaluation
value at each lens position and determine a lens position with the
maximal AF evaluation value as in-focus position.

[0045] The AE and AWB evaluation values are calculated from the integrated
values of RGB values of the RAW-RGB data. For example, the image frame in
association with all the pixels of the CMOS 104 are equally divided into
256 areas (16 by 16 horizontally and vertically) to calculate the
integration of the RGB values of each area.

[0046] In AE process, the signal processing unit 101 calculates brightness
of each equally divided area using the RGB integration values to
determine a proper exposure amount on the basis of a calculated
brightness distribution and set an exposure condition as electric shutter
speed and f-value according to the exposure amount.

[0047] Further, in AWB process the signal processing unit 101 determines a
control value for the color of a light source of a subject according to a
distribution in the RGB integration values, and the YUV converter
converts the RAW-RGB data into YUV data while adjusting the white balance
thereof.

[0048] The AE operation and AWB operation are repeated during the preview
operation. Upon a full press to the shutter button, the preview operation
is switched to still image shooting operation.

[0049] Triggered by the detection of a full press to the shutter button 2,
the still image shooting is started. First, the CPU 102 reads an image
from the sub optical system 16 to calculate the distance to a subject.
Then, it instructs the motor driver 103 to move the focus lens of the
imaging lens 5 to a certain position in accordance with the calculated
subject distance.

[0050] Then, the focus lens is moved in a certain area to calculate the AF
evaluation value, and moved to an in-focus position with the maximal AF
evaluation value. The above-described AE operation follows. Upon the
completion of an exposure, the CPU 102 instructs the motor driver to
close the mechanical shutter of the unit 18 and the CMOS 104 to output
analog RGB image signals for still image which are converted into RAW-RGB
data as in the preview operation.

[0051] The converted RAW-RGB data is captured into the CMOS I/F of the
signal processing unit 101, converted to YUV data by the YUV converter,
and stored in the memory 108 via the memory controller. The YUV data is
read from the memory 108 at certain timing, converted by the resize unit
to a suitable size for the number of recorded pixels and compressed in
JPEG form or else by the data compressor.

[0052] The compressed image data is written to the memory 108 again, read
therefrom via the memory controller, and stored in the memory card 14 via
the medium I/F.

[0053] The sub optical system 16 can output image data based on a subject
image formed on a not-shown ranging element via a ranging lens. The
signal processing unit 101 reads the image data from the sub optical
system 16 and calculates the distance to the subject from this image
data. The focus lens is moved to a position in accordance with the
calculated distance, and moved around the position in question to find an
in-focus position. Thereby, the in-focus position can be calculated
without moving the focus lens in the entire driving area.

[0054] In the present embodiment, the sub optical system 16 cannot perform
zoom operation. According to the present embodiment, the angle of view of
the image data acquired via the ranging lens of the sub optical system 16
is assumed to be equivalent to that of image data obtained by the main
optical system 17 when the zoom lens thereof is positioned at wide angle
end.

[0055] When the main optical system 17 performs zoom operation, the angle
of view of the sub optical system 16 does not coincide with the angle of
view of the main optical system 17. In such a case the association
between the image data obtained by the sub optical system 16 and that
acquired by the main optical system 17 is found by template matching or
the like. Then, the distance to a subject is calculated and a
later-described sub image is displayed using the image data of an area
from the sub optical system 16 associated with that from the main optical
system 17.

[0056] The digital camera 1 can read preview image data from the sub
optical system 16 instead of the main optical system 17. That is, to
heighten the AF process speed, it is configured to perform the preview
operation using image data from the sub optical system 16 when stopping
using image data from the main optical system 17. Thereby, the digital
camera 1 can easily track the motion of a subject.

[0057] Herein, an image displayed on the LCD 9 based on image data output
from the sub optical system 16 is referred to as a sub image. In the
following the sub image display function of the digital camera 1 is
described with reference to FIG. 3.

[0058] The digital camera 1 starts operating in the shooting mode and
displays a preview image in step S1. A determination is made on whether
or not the shutter button 2 is half-pressed in step S2. Until the shutter
button 2 is half-pressed (RL1), the preview display is continued (No in
step S2):

[0059] In step S3 the AF process starts upon detection of the half-press
to the shutter button 2 (Yes in step S2). The AF process is described in
detail referring to FIG. 4.

[0060] First, an external AF operation is performed using the sub optical
system 16 in step S31, to calculate a parallax in the image data of a
subject obtained by the two image sensors of the sub optical system 16
and calculate the distance to the subject by triangulation using the
parallax. The CPU 102 moves the focus lens to a position in accordance
with the calculated distance in step S31.

[0061] Next, in contrast autofocus operation in step S32, the focus lens
position with the maximal contrast value is determined while the focus
lens is moved in a certain area around the position found in the step
S31. The details of the contrast autofocus operation are described with
reference to FIG. 5.

[0062] The contrast autofocus operation uses a partial image based on a
part of the image signal from the main optical system 17. Because of
this, the display of a main image on the LCD 9 is temporarily stopped in
S321. The main image refers to an image which is displayed on the LCD 9
during the preview, using image data from the main optical system 17
(step S1 in FIG. 3). In step S321 the image display on the LCD 9 at the
start of the contrast autofocus operation in step S32 is frozen or the
display of an image read from the main optical system 17 is stopped.

[0063] Next, in step S322 a determination is made on whether or not to
display a sub frame. If the contrast AF operation based on the image data
of not a partial image area but the entire image area is expected to be
completed in less than a predetermined period of time, the display of the
sub frame is not effective and hard to view since the sub frame is
switched over the main frame quickly in a short time. Further, if the
contrast AF based on the partial data read from the main optical system
17 is expected to be completed in a predetermined period of time, it is
better not to display the sub frame for the purpose of preventing the
quick display switchover.

FIRST EXAMPLE

[0064] There are several ways of determining whether to execute the sub
frame display in step S322. For example, with the focal length of the
main optical system 17 at wide angle end, the number of scan steps for
the contrast autofocus is fewer than at telephoto end due to a large
depth of field. In this case the termination of the preview of the main
image can be ended in a short time so that the display of the sub frame
is not necessary since a subject is unlikely to move out of the display
area in that time. The switchover to the sub frame rather interrupts the
image display. Accordingly, when the focal length of the main optical
system 17 is lower than a preset value, the sub frame is not displayed
even during the termination of the main image display based on the
partial read AF (No in step S322). When the focal length of the main
optical system 17 is higher than the preset value, the flow proceeds to
sub frame display process in step S323 (Yes in step S322).

SECOND EXAMPLE

[0065] The operation in step S322 is, in other words, to determine a
period of time in which the preview of the main image is stopped, in
accordance with the angle of view of the main optical system 17. With the
angle of view at wide angle end and a large depth of field, the stop
period can be short since the number of scan steps is less than that at
telephoto end. Thus, when the angle of view of the main optical system 17
is higher than a preset threshold, the sub frame display is not performed
(No in step S322). When the angle of view of the main optical system 17
is lower than the preset threshold, the flow proceeds to step S323 (Yes
in step S322).

THIRD EXAMPLE

[0066] Further, at the number of scan steps being less than a
predetermined value, the sub frame display is not started (No in step
S322) while at the number of scan steps being more than the predetermined
value, the flow proceeds to step S323.

[0067] In the digital camera 1 having a scene selection function in which
an exposure condition or a shutter speed is selectable from multiple
options in shooting mode, it is possible to reduce the number of scan
steps for the contrast autofocus according to a selected scene or a
result of the most previous AF process.

[0068] Therefore, the number of scan steps is not always small at wide
angle end nor is it always large at telephoto end. At a small number of
scan steps for the contrast AF, the freeze of the main image ends in a
short time, so that the display of the sub frame is unnecessary since a
subject is unlikely to move out of the display area in the short time.
The switchover between the sub frame and the main frame in a short time
rather hinders the viewablity of image display.

[0069] Accordingly, when the number of scan steps to be executed is
smaller than a preset value, the sub frame display is not performed even
during the stop of the main image display based on the partial read AF
(No in step S322) while when it is larger than the preset value, the sub
frame is displayed (Yes in step S322, step S323).

FOURTH EXAMPLE

[0070] Further, the display of the sub frame can be determined depending
on an estimated contrast AF time. With the estimated contrast time being
shorter than a predetermined time, the sub frame display is not started
(No in step S322) while with the estimated contrast time being longer
than the predetermined time, the flow proceeds to the step S323 (Yes in
step S322).

[0071] To increase the contrast AF speed, the frame rate for calculating
the contrast value has to be heightened. This makes it possible to read
images at a high frame rate even if the number of scan steps of the focus
lens is large, and to shorten the contrast AF time. Meanwhile, even with
a small number of scan steps, the contrast AF time becomes longer at a
low frame rate. In general the image sensor can be driven at a high frame
rate when the brightness of a subject is high while it has to be driven
at a low frame rate when a subject has a low brightness and requires a
long exposure time.

[0072] Thus, the contrast AF time can be estimated from the number of scan
steps and the frame rate of the main optical system 17 to determine
whether or not to display the sub frame during the stop of the main frame
based on the partial read AF (No or Yes in step S322 and step S323).

[0073] As described above, upon determining the non-display of the sub
frame based on the above conditions (No in step S322), the flow proceeds
to the determination on completion of the contrast AF in step S324. Upon
determining the display of the sub frame (Yes in step S322), the flow
proceeds to step S323.

[0074] An example of the sub frame display in step S323 is described in
detail with reference to FIG. 6. In step S3231 image data is read from
the sub optical system 16. The image can be read from either of the two
image sensors of the sub optical system 16.

[0075] In step S3232 the read image data is converted to one in a size
suitable to superimpose over the main image by the signal processing unit
101. The resized image data is referred to as a sub image. In step S3233
the sub image is synthesized with the main image.

[0076] In step S3234 the synthesized image is output to the LCD 9. Thus,
by superimposing the sub image over the frozen main image on the display,
the subject can be tracked and captured in the display area during the
contrast AF process.

[0077] Next, an example of the image display on the LCD 9 of the digital
camera 1 in the sub frame display process in step S322 is described
referring to FIG.7. FIG. 7 shows an example of the sub frame on the LCD
9, in which a person X1 and a tree X2 are captured as a subject. On the
LCD 9 displayed are a main image 100 read via the main optical system 17
and a sub image 101 read via the sub optical system 16.

[0078] During the contrast AF operation of the digital camera 1, the main
image 100 is placed in a frozen state although the condition of the
subject may change in that time. If the person X1 is moving away from the
tree X2, for example, the sub image 101 can show the current state of the
subject in sequence since it is updated at a predetermined frame rate
unlike the frozen main image 100.

[0079] Thus, the imaging device according to the present embodiment is
able to continue to display the preview images of the subject even during
the contrast AF operation.

[0080] Next, another example of the image display on the LCD 9 in the sub
frame display process in step S323 is described with reference to FIGS.
8A, 8B.

[0081] In FIG. 8A a user is allowed to select, with a not-shown setting
menu, a display position of a sub image from positions 101a (top right),
101b (top left), 101c (bottom left). By setting the display position of
the sub image, a synthesized image is generated in the image synthesis
process in step S3233.

[0082] As shown in FIG. 8B, a sub image 101d can be almost as large as the
main image 100. Also, only the sub image 101 can be displayed without the
main image 100.

[0083] Returning to FIG. 5, in step S324 a determination is made on
whether or not the contrast AF operation is completed, that is, the focus
lens has been moved to a certain position in accordance with the maximal
contrast value. With the contrast AF not completed (No in step S324), the
flow returns to step S322 to determine whether or not to perform the sub
frame display again.

[0084] Upon the completion of the contrast AF (Yes in step S324), the flow
returns to step S33 in FIG. 4 to display the in-focus indication on the
subject image on the LCD 9. The in-focus indication refers to an
indication to notify a user that the AF process is completed and the
camera is now ready for shooting.

[0085] Returning to FIG. 3, after the AF process in step S3, a
determination is made on whether or not the shutter button 2 is
full-pressed (RL2) in step S4. When the full-press to the shutter button
2 is detected (Yes in step S4), a shooting operation is conducted in step
S5. Upon completion of the shooting, the preview operation is resumed in
step S6.

[0086] In step S6 the image preview on the LCD 9 is resumed by resuming
reading images from the main optical system 17 and stopping reading
images from the sub optical system 16.

[0087] Upon detection of the half-press (RL1) (Yes in step S7) instead of
the full-press (RL2) (No in step S4), the in-focus indication is
continuously displayed. With no detection of RL1 (No in step S7), the
image preview is resumed in step S6.

[0088] As described above, the imaging device according to the present
embodiment can continuously display a subject image on the LCD 9 without
stopping the image preview, while partially reading image data in order
to perform the contrast AF operation at a higher speed.

[0089] Moreover, the imaging device according to the present embodiment
can easily track the motion of a subject in the contrast AF process.

[0090] Although the present invention has been described in terms of
exemplary embodiments, it is not limited thereto. It should be
appreciated that variations or modifications may be made in the
embodiments described by persons skilled in the art without departing
from the scope of the present invention as defined by the following
claims.